Cellulose is an important industrial raw material and a source of renewable energy. The physical structure and morphology of native cellulose are complex and the fine details of its structure have been difficult to determine experimentally. However, the chemical composition of cellulose is simple, consisting of D-glucose residues linked by beta-1,4-glycosidic bonds to form linear polymers with chains length of over 10,000 glycosidic residues.
In order to be efficient, the digestion of cellulose requires several types of enzymes acting cooperatively. At least three categories of enzymes are necessary to convert cellulose into glucose: endo (1,4)-beta-D-glucanases (EC 3.2.1.4) that cut the cellulose chains at random; cellobiohydrolases (EC 3.2.1.91) which cleave cellobiosyl units from the cellulose chain ends and beta-glucosidases (EC 3.2.1.21) that convert cellobiose and soluble cellodextrins into glucose. Among these three categories of enzymes involved in the biodegradation of cellulose, cellobiohydrolases are the key enzymes for the degradation of native crystalline cellulose.
Exo-cellobiohydrolases (Cellobiohydrolase II or CBH II) refer to the cellobiohydrolases which degrade cellulose by hydrolyzing the cellobiose from the non-reducing end of the cellulose polymer chains. The cellobiohydrolase II group belongs to the same EC group, i.e., EC 3.2.1.91, as the cellobiohydrolase I group, the difference being that cellobiohydrolase I degrade cellulose by hydrolyzing the cellobiose from the reducing end of the cellulose polymer chains.
It is an object of the present invention to provide improved polypeptides having cellobiohydrolase II activity and polynucleotides encoding the polypeptides. The improved polypeptides may have improved specific activity and/or improved stability—in particular improved thermostability. The polypeptides may also have an improved ability to resist inhibition by cellobiose.